The invention relates to a process for the internal high-pressure forming of a workpiece in a closed internal high-pressure forming die, the workpiece being expanded on account of the fluidic internal high pressure exerted by a pressure generator and being brought to bear against a cavity of the forming die. The invention also relates to an apparatus for carrying out the process which has an internal high-pressure forming die which is divided along the extent of the workpiece and has a pressure generator which is fluidically connected to the forming die for exerting a fluidic internal high pressure on the workpiece.
A process of the generic type and an apparatus of the generic type are known from German Patent Document DE 94 07 812.2 U1. In this case, a hollow profile forming the workpiece is placed into an internal high-pressure forming die divided along the extent of the hollow profile, after which the die is closed. The hollow profile is subsequently closed in a sealing manner at both ends by axial male dies. The space inside the hollow profile is filled by means of the axial male dies. The axial male dies are fluidically connected to a pressure generator. Then, an internal high pressure is applied within the hollow profile by means of the pressure generator, having the effect of expanding the profile until it is brought to bear against the wall of the die cavity. The die cavity has a branch which leads away radially from the extent of the hollow profile and into which the material of the hollow profile is forced by being subjected to the internal high pressure, with the effect of forming a neck. Here, too, the material is brought to bear against the wall of the branch. The expanding process within the branch is stabilized by a counterslide which is guided displaceably in the branch and supports the neck in the end region. To extend the failure limits with regard to bursting during pure expansion, in which the length of the hollow profile is shortened, and consequently to obtain a greater expanded length of the neck, further material of the hollow profile is pushed towards the branching location by an additional axial force, which is applied by moving in of the axial male dies, with the effect of at least partially compensating for the thinning of the material in the branching region responsible for bursting. Nevertheless, when the material of the hollow profile is in contact with the cavity wall, in particular also in the branch, the shaping capability of the material is limited considerably by the friction produced between the die and the hollow profile, which becomes greater with increasing internal high pressure--even if the hollow profile is provided with a lubricant on its outer side. This is very problematical most particularly in the case of materials with little propensity to undergo forming.
The invention is based on the object of developing a process and an apparatus in which process reliability in the internal high-pressure forming of workpieces is improved.
The object is achieved according to the invention with regard to the process by introducing an internal high pressure in the form of a pressure oscillation. The apparatus of the present invention achieves the object by incorporating a pressure oscillator by means of which the internal high pressure can be introduced into the workpiece as a pressure oscillation.
The fact that the internal high pressure acts on the workpiece in the form of a pressure oscillation has the effect that the friction of the workpiece on the cavity wall is reduced during the period of time between the maxima of the oscillation, since there is a region of lower pressure there. In other words, during the forming operation there is a periodically occurring partial relief of the normal stress of contact between the workpiece and the die. By reducing the friction, however, the resistance to additional material of the workpiece flowing or being fed towards the shaping location also becomes less, so that more material can be moved to this location without failure. This counteracts thinning of material in the shaping region, a more favorable distribution of material for forming being achieved and/or it being possible to increase the degree of forming. For example in the case of a branch from the cavity, i.e. when forming a neck, this may mean an increase in the expanded length. Consequently, the limits of the process can be extended, for example with regard to the production of secondary formed elements. Furthermore, on account of the increased feeding of additional material, narrower radii can also be formed on the workpiece in a reliable process, without cracking. Altogether, the invention achieves improved forming even of workpieces from materials with little propensity to undergo dimensional change. Partial isolation of the process parameters from the tribological conditions accomplishes greater process stability. For designing the process optimally with the greatest possible benefit for the respective forming, the pressure oscillation is to be adapted appropriately in its amplitude and frequency with regard to the material of the workpiece and the degree of forming of the shape to be accomplished, in other words the geometry of the fully formed workpiece or other relevant process parameters.